XF 1.1Avatars not displaying in Thread list

Member

I recently changed the default avatars, and now I've noticed that the "unknown" avatar is displaying in thread lists (eg. here) even when the user has an avatar. The real avatars still exist, and still display everywhere else (main forum, profile pages etc). Also, when there are photos in the first post of a thread, those display in place of the avatar (eg here).
Why might this be? I haven't (deliberately) changed any permissions or altered anything other than the default avatar images.
I'm on 1.1.5 and using Last Post Avatar by Waindigo, (plus other addons not related to avatars) - disabling addons doesn't seem to fix the problem.